1. Field of the Invention:
This invention relates to a process for preparing alcohols by catalytic hydration of olefins and more particularly, to a process for preparing alcohols by hydration of aliphatic olefins having from 2 to 12 carbon atoms in the presence of a corrosion preventive agent of a specific type.
2. Description of Prior Art:
A number of processes for preparing alcohols by direct hydration of olefins in the presence of catalysts are known. Typical of the processes are a process for hydrating olefins in the presence of heterogeneous catalysts such as of metal oxides and a process for hydrating olefins in the presence of homogeneous catalysts such as of aqueous inorganic acid solutions including sulfuric acid. It is also known that these hydration reactions are feasible either in liquid phase or in gas phase.
The liquid phase process known such as in British Pat. No. 1,281,120 and using a heteropoly-acid catalyst has the following advantages and disadvantages in comparison with the gas phase process using a phosphoric acid-on-carrier catalyst or a so-called solid phosphoric acid catalyst and known from old such as in U.S. Pat. No. 2,232,610. The liquid phase process is advantageous in that it can yield relatively high conversion rate of olefins, so that inexpensive olefins with relatively low purity are usable as starting material, but disadvantageous in that a strongly acidic catalyst is used in the form of an aqueous solution, so that a reactor or other apparatus is attacked and corroded to a considerable extent by the action of such acidic catalyst.
The corrosion tendency is likewise experienced in a liquid phase process using a phosphoric acid catalyst in the form of an aqueous solution. The phosphoric acid catalyst which is known to have relatively high hydration activity such as disclosed in Canadian Pat. No. 845,202 is a highly corrosive acid and has never been employed industrially since there is found no corrosion preventive material suitable for the above purpose. It will be noted that, in the Canadian Patent, a ceramic lined reactor is used for experiments in a laboratory scale production.
There have been studied and developed a variety of corrosion preventive agents so as to suppress the corrosion of chemical plants or apparatus. However, few of them stand use at high temperatures of above 200.degree. C. For example, deposition film-forming corrosion preventive agents such as polyphosphoric acid salts and zinc salts show a good corrosion preventive ability only when employed at a relatively low temperature below 100.degree. C. While, adsorption film-forming corrosion preventive agents such as some amines and surface active agents are believed that a critical temperature sufficient for giving a stable corrosion preventive effect is in the range of 150 - 160.degree. C at a maximum when considered on an adsorption equilibrium basis (see the Material of the Eleventh Course In Corrosion and Corrosion Prevention, page 225 (1972) under the promotion of Japan Association of Materials). Further, oxidation film forming corrosion preventive agents such as chromates, molybdates and tungstates have been widely used as neutral corrosion control agents but have hardly employed against aqueous acidic solutions. These agents are considered to be reduced by the action of organic reducing materials and lose their control action (see the Hand-book of Metal Corrosion Prevention Technology, page 333 (1961) complied by the Japan Learning and Study Advancement Society). Thus, there are not yet found any corrosion preventive agents which can stand use even in a reducing atmosphere which will be experienced, for example, upon the direct hydration of olefins by a liquid phase process.